AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

How Aircraft Engines and Propellers Connect and Function

While modern aircraft now typically rely on powerful gas turbine engines to create the thrust and power necessary for flight, there are still countless models that feature more conventional assemblies like reciprocating engines and propeller blades. With this pair, power from combusted fuel can be harnessed by propeller assemblies to create a forward thrusting force, allowing the aircraft to overcome drag for movement and flight. 

As fuel and air are mixed, distributed, and ignited within pistons, the engine will generate rotational motion through a crankshaft which itself is connected to propeller blades through a number of reducing gears and elements. This rotational force then turns the propeller blade assembly, allowing it to spin through the air like a screw to generate a forward force. As the propeller operates in unison with the aircraft engine, the pair must be suitable so that performance is upheld and remaining airborne is achievable without struggle.

The shape of propeller blades is quite comparable to that of an aircraft’s wings, generally being an airfoil shape that is twisted and rotated. All blades extend from a centralized propeller hub, chords joining leading edges and trailing edges of each. As the propeller blades are rotated perpendicularly to the motion of the aircraft itself, the force they create allows for forward speed instead of upward lift. Like wings, propeller blades will also generate drag force that must be overcome for the sake of efficiency.

When propeller designs are initially created for aircraft, technicians will generally be concerned with variables like diameter, blade number, and propeller pitch. When discussing diameter, it is important to understand that increased diameter will generally cause a higher amount of thrust to be created during rotation. As such, the more thrust an aircraft needs, the larger propeller blade diameters will be. There is a limit to this though, as diameter values will often alter properties related to blade stress, torque requirements, clearances, etc.

In terms of blade count, such values are often determined based on the engine installed on an aircraft, larger models needing more blades to account for increased power production. To ensure that power production and thrust generation is efficient without a considerable amount of loss, equations like the solidity ratio may be used. As such, aircraft may feature anywhere from three to eight blades on each engine assembly.

As the final consideration, pitch determines the ease at which a propeller may rotate within the air, and it is largely determined by the angle of blades. Fine-pitch propellers are those with low blade angles, and they are able to spin quickly as short distances forward are covered with each revolution. Meanwhile, coarse-pitch propellers cover larger distances each revolution, taking a large “bite” out of the air with each turn as a result of the greater blade angle. Blade pitches may be adjustable or fixed based on the type of propellers one has on their aircraft.

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